1. Field of the Invention
The present invention relates to an optical multiplexer/demultiplexer for multiplexing/demultiplexing lights of different wavelengths, and in particular, to an optical multiplexer/demultiplexer using a diffraction grating.
2. Description of the Related Art
As a conventional optical multiplexer/demultiplexer, there has been known a configuration using a prism or a diffraction grating as disclosed in Japanese Unexamined Patent Publication No. 55-29824 or Japanese Unexamined Patent Publication No. 8-5861. Further, there has been known a configuration using a multi-reflection layer as disclosed in Japanese Unexamined Patent Publication No. 10-319256.
FIG. 6 is a top plan view showing a waveguide configuration using a concave diffraction grating as one example of conventional optical multiplexer/demultiplexer using a diffraction grating. To be specific, the optical multiplexer/demultiplexer in FIG. 6 comprises: a slab waveguide 101; a single input waveguide 102 connected to the slab waveguide 101; an output waveguide group 103 consisting of one or more output waveguides connected to the slab waveguide 101 on the same side of the input waveguide 102; and a reflection concave diffraction grating 104 disposed to the slab waveguide 101 on the opposite side of the input waveguide 102. In this optical multiplexer/demultiplexer, if the total length in a lengthwise direction of the slab waveguide 101 is f, a shape of one end face of the slab waveguide 101 is an arc of diameter f passing through a center point P0 of the slab waveguide 101, and the input waveguide 102 and the output waveguide group 103 are connected onto the arc. Further, a shape of the other end face of the slab waveguide 101 is an arc of radius f, whose center is a point P1 on the one end face positioned between the input waveguide 102 and the output waveguide group 103, and the concave diffraction grating 104 is formed on this arc.
In the conventional optical multiplexer/demultiplexer having the above configuration, lights incident from the input waveguide 102 are freely propagated through the slab waveguide 101 to be reflected by the concave diffraction grating 104. At this time, the lights reflected by respective grooves of the concave diffraction grating 104 interfere with each other, to be diffracted to a direction where an optical path length difference between adjacent lights becomes an integral multiple of the wavelength (to be referred to the diffraction order). Generally, provided that an angle (incident angle) of a propagation direction of an incident light to a normal on a grating plane of the diffraction grating is α, and an angle (diffraction angle) of a propagation direction of a diffracted light to the normal marked on the grating plane of the diffraction grating is θ, a relationship shown in the following equation (1) is established between the incident angle α and the diffraction angle θ.
                                          sin            ⁢                                                  ⁢            θ                    -                      sin            ⁢                                                  ⁢            α                          =                              m            ·            λ                                              n              s                        ·            d                                              (        1        )            Note, m is the diffraction order, λ is a center wavelength of the incident light, ns is the effective refractive index of the slab waveguide, and d is a grating interval of the diffraction grating.
Normally, for the intensity of lights reflected by the diffraction grating, as shown in (A) of FIG. 7, a reflected light LR of a light L1 incident on the diffraction grating has the highest intensity. Therefore, as shown in (B) of FIG. 7, the inclination of the reflection plane (groove plane) of the diffraction grating to the incident light L1 is designed so that a propagation direction of a reflected light LR becomes equal to a propagation direction of a diffracted light LP of required order m, and the diffraction grating is blazed so that the energy of the incident light L1 to a specific wavelength is effectively converted into the diffracted light LP. In such a blazed diffraction grating, an angle ε between a normal of the grating plane and a normal of the reflection plane becomes a blaze angle, and a wavelength diffracted to a direction equal to the normal of the reflection plane becomes a blaze wavelength. The blazing of the diffraction grating as described above has been conventionally performed only for the diffracted light of a single direction (order).
Generally, the optical multiplexer/demultiplexer using a diffraction grating has a drawback in that, if the wavelength spacing of lights to be demultiplexed (or multiplexed) is narrowed, since a focal distance of a diffracted light is lengthened, the size of the optical multiplexer/demultiplexer is enlarged. To be specific, a focal distance f of the optical multiplexer/demultiplexer using the concave diffraction grating 104 as shown in FIG. 6 can be simply calculated in accordance with the next equation (2).
                    Δλ        =                                            d              ·              Δ                        ⁢                                                  ⁢                          x              ·                              n                n                            ·              cos                        ⁢                                                  ⁢            θ                                m            ·            f                                              (        2        )            Note, Δx is an interval of the output waveguides, Δλ is the wavelength spacing of the lights to be demultiplexed (or multiplexed).
The consideration will be made on, for example, an optical multiplexer/demultiplexer having the wavelength spacing Δλ1 and an optical multiplexer/demultiplexer having the wavelength spacing Δλ2 which is half the wavelength spacing Δλ1 (=Δλ1/2). Here, a focal distance for when the wavelength spacing is Δλ1 is f1, and a focal distance for when the wavelength spacing is Δλ2 is f2. In this case, if the same diffraction order m, grating interval d and output waveguide interval Δx are used in each optical multiplexer/demultiplexer, the focal distance f2 becomes twice the focal distance f1 according to the equation (2). In order to reduce the focal distance f2, it is necessary to reduce either a value of the grating interval d or a value of the output waveguide interval Δx, or to increase the diffraction order m.
However, if the output waveguide interval Δx is narrowed, the optical coupling between adjacent output waveguides is strengthened, resulting in the degradation of adjacent crosstalk. On the other hand, if the diffraction order m is increased, since a FSR (free spectrum region) of the optical multiplexer/demultiplexer is narrowed, there is a problem in that an optical intensity difference between output channels is increased.
Further, in the case where the grating interval d is narrowed or the diffraction order m is increased, it is apparent from the relationship in the equation (1) that the diffraction angle θ is enlarged. Since the diffraction angle θ cannot be made to be 90° or more, there is a certain limit to the optimization of the grating interval d or the diffraction order m to reduce the focal distance f2.
The present invention has been accomplished in view of the above problems and has an object to provide an optical multiplexer/demultiplexer of small size, capable of multiplexing/demultiplexing lights of narrow wavelength spacing at a short focal distance.